1. Field of the Invention
This invention relates to an illuminating device for image exposure in an electrophotographic copying machine, an image reader and the like, which employs a fluorescent lamp whose voltage is variable as an illumination source and, more specifically, to an illuminating device which is capable of effectively starting up the illumination and of indicating an error status when the light amount for illumination is insufficient after preliminary lighting is performed for a fixed amount of time.
2. Description of the Prior Art
The amount of light emitted from a fluorescent lamp of an illuminating device in an electrophotographic copying machine fluctuates according to the tube wall temperature of the lamp. Thus, when copying is conducted under low room temperature, for example in winter, a conventional copying machine is slow in getting started because the tube wall which is initially equal to room temperature needs to be heated in order to obtain a desired amount of light for exposure. Illuminating devices of prior art, such as that disclosed in Japanese Laid-Open Patent Application No. 61-140933, attempt to overcome this disadvantage and to get ready to make copies in a shorter time period by increasing the tube wall temperature of the fluorescent lamp through heat which is generated by lighting the lamp at the maximum allowable voltage for a fixed amount of time immediately after the power is turned on.
However, the preliminary lighting is performed for a fixed amount of time which is irrespective of either initial tube wall temperature or desired light amount, and thus, the tube wall is not always heated to an appropriate temperature. Also, in case the tube wall temperature has not reached the appropriate temperature there is no way to know why; it may be due to low initial temperature of the tube wall, to exhaustion of the lamp, or to any other trouble of the copying machine. If the user keeps on using a copying machine with an illuminating device in the latter two cases, much serious troubles may be caused in the machine. Furthermore, the preliminary lighting at maximum allowable voltage reduces the life span of the fluorescent lamp.